1. Field of the Invention
The present invention relates to open-air fields for use as athletic fields, ballgame fields or playgrounds.
Open-air fields, i.e. those having no means for excluding rain or snowfall and yet can be used for athletics, ballgames etc. under all normal weather conditions must meet two basic requirements: they must be capable of being used even in rain showers, or very shortly after a heavy rain fall, substantially without formation of water puddles on the field and without the surface losing its tread-resistant properties. The term "tread resistance" as used herein indicates the property of a field surface to withstand being tread upon by the feet of athletes, players or mounts without substantial damage and without impairing normal motion of players, animals or sporting devices.
Further, while such surfaces must be tread-resistant, they should not cause injuries to players falling to, or sliding on, the ground by accident or intention. Firstly, this implies selective resilience of the top layer of the field in the sense of softening the impact of a falling player while providing sufficient rebound to the feet of running players or horses, as well as to sporting devices, such as pneumatic balls. Secondly, this implies a relatively low surface friction in the sense that the unprotected skin of a player sliding along the ground under a forward momentum will be neither abraded, as is the case with sand or ash surfaces, nor burnt, as is the case with some plastic surfaces.
Typical games that would benefit from fields that satisfy these requirements include football, field handball, soccer, baseball, rugby, hockey, polo and other games conventionally played by opposing teams on open-air fields. Generally, the term "athletic field" as used herein is intended to include fields which are, or can be, used temporarily or exclusively for ball games of the types indicated above including team training.
2. Description of the Prior Art
Two main categories of open-air fields are defined by the mechanisms involved in getting rid of water that might accumulate at the upper surface of the field: the top layer either prevents or permits vertical passage of water. Hence, the top layer of the former category is substantially impermeable to water, e.g. due to an essentially continuous matrix or stratum of a water-impervious material, such as bitumen or plastics, and the surface water is removed by surface flow caused on account of a light inclination of the surface by means of water-collecting channels and openings in the surface or in lateral areas thereof. The second or water-permeable type of top layers of athletic or ball game fields
permits an essentially instantaneous vertical passage of surface water, i.e. at a vertical flow rate that is commensurate with normal rain intensity, expressed, for example, in milliliters per square meter per minute; in this second category, water puddles must not form, or must disappear within minutes or even seconds after a heavy shower. By the same token, water produced by melting of a snow layer on top of the field's surface must be able to permeate the field's top layer without substantial accumulation near the surface since this would promote microbiological growth on, or decay of, components of the top layer.
The present invention is concerned with the second or water-permeable type of top layers. Since the water permeability of the top layer is but a part of the overall permeability of a normally multi-layered open-air field structure, it goes almost without saying that the sub-stratum below the top layer must be capable to absorb or pass water at the maximum rate of its permeation through the top layer. This, however, is conventional and can be achieved by drainage systems, e.g. perforated pipes arranged in a sub-stratum and communicating with a suitably dimensioned outlet and/or pumping mechanism.
Conventional water-permeable top layer structures include grass fields both of the natural as well as the synthetic type; while natural grass has some self-repair capacity, this requires time and care. Generally, natural grass fields do not support continuous (e.g. daily) use and require a very substantial maintenance effort. Hence, maintenance costs and allowable use intensity tend to severely limit their application for heavy use.
Fields of synthetic grass, i.e. those formed of man-made polymer structures resembling the structure of natural grass, on the other hand, require less maintenance and support heavy use but are very costly as regards production. Further, they tend to cause severe abrasions or burns of unprotected skin portions of players that slide on the field after a fall, tackle or the like ground contact.
The third general category of water-permeable open-air fields has a top layer formed of an essentially granular material, such as sand or mineral ash, in a more or less densely compacted form. Obviously, the degree of tread resistance and hardness will depend upon the particular use and a tennis field, for example, may require a somewhat different surface quality than a field used for soccer training.
Basic general structures are defined, for example, in German Industrial Standards (DIN 18035/1973) and such standards may be consulted not only for proper selection of conventional mineral top layers but also for suitable substrata structures of use for organic top layers of the type contemplated by the present invention.
Generally, conventional "sand-top" or "hard-top" mineral top layer structures (German: "Tennenplatze") for athletic or ball game fields are less expensive in construction and maintenance but have the severe common disadvantage of causing bone fractures, contusions and large abrasions when used for athletics or ball games which involve much ground contact of the players' bodies
As a consequence, fields with mineral top layers are sometimes used for ball game training but actual competitions are generally played but on natural or synthetic grass fields.
Sporting surfaces for ski slopes in the form of undercovering layers beneath natural or artificial snow have been disclosed in U.S. Patent No. 3,427,934 to Zames who suggests to use particulate tree bark for forming a cover on an underlying sub-stratum of grass and rocks to reduce the degree of natural irregularities for a ski track that will be formed by the snow cover on the tree bark layer. Tread resistance is not an issue here, however, nor is water permeability.
Further, tree bark material is conventionally used as a top layer material for jogging tracks, horse training grounds and childrens' playfields but is not, per se, normally suitable for athletic or ball game fields that require a substantial tread resistance. However, tread resistance can be increased by ordered deposition of a generally platelet-shaped tree bark material and/or by the use of permanent or temporary binding agents including water.
A top layer of particulate tree bark for soccer fields with improved tread resistance is disclosed in Applicants' European Patent Specification No. 0 096 908 involving a two-layered top structure formed of tree bark materials of differing particle sizes including a lower layer having a coarser texture and an upper layer having a finer structure.
While natural components of tree bark, notably tannins, tend to reduce or minimize natural decomposition of the particles under ambient conditions and in the presence of natural microbiological organisms and humidity, such decomposition cannot be totally precluded in practice, not even when using additional stabilizing agents because the latter would be leached out eventually, aside from undesirable environmental effects of such additives. Natural biological decomposition of an organic material is generally termed "rot" and the resistance of a given material against such decomposition is termed "rot resistance" herein. Specific methods for quantifying this parameter are known in the art and will be briefly explained in connection with the examples herein. By the same token, the extent to which a given organic material has been decomposed by rotting will be termed "rot degree" (German: "Rottungsgrad") herein; an approximative quantification of this parameter for tree bark can be given by means of color grading: fresh tree bark is brownish to maroon and becomes gradually darker as humic acids are formed by decomposition. Further, as a consequence of the combined effects of mechanical impact upon continued use, and of natural decomposition, tree bark particles of a top layer tend to become smaller by attrition. As a consequence, the interstices of a layer of coarse tree bark particles will tend to become filled by finer particles upon prolonged use so as to increase packing density and reduce water permeability. Accordingly, it has been assumed that an athletic field or playground having a tread-resistant top layer formed of tree bark particles would require replacement of the degraded tree bark material eventually.